1. Field of the Invention
The present invention relates to a surface-mounted light emitting device used in lighting apparatus, display, backlight light source for cell phone, auxiliary light source for moving picture and other light sources for general use, and resin molding suitable for the same.
2. Description of the Related Art
Surface-mounted light emitting devices based on the light emitting element are small in size, high in power efficiency and are capable of emitting light of clear colors. The light emitting element that is a semiconductor element is also free of failure such as burnout. Moreover, the light emitting element has such advantages as excellent startup characteristic and high durability to vibration and to repetitive operations of turning ON/OFF. Because of such excellent characteristics, the light emitting devices based on light emitting elements such as light emitting diode (LED) and laser diode (LD) are used in various applications of light source.
The conventional surface-mounted light emitting device comprises a light emitting element, a mounting lead frame whereon the light emitting element is mounted, a connection lead frame to be connected via a lead wire to the light emitting element and a package that covers most parts of the lead frames (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-87780). With regard to the surface-mounted light emitting device, there has been a case where the package is formed by a molding thermoplastic resin (for example, liquid crystal polymer, PPS (polyphenylene sulfide), nylon, etc.) having favorable properties for molding. The thermoplastic resin used to form the package is generally required to have heat resistance so as to endure the high temperature that is caused when melting a reflow solder. Accordingly, engineering polymers such as semi-aromatic polyamide, liquid crystal polymer and PPS, that have relatively high heat resistance among the thermoplastic resins, have been used.
With the recent trend of increasing output power of light emitting devices, heat generated during light emission has been increasing at a remarkable pace. As a result, the package is heated to a high temperature by the heat generated by the light emitting element housed in a recess, and the conventional package formed of a resin has been undergoing discoloration and/or deformation due to the heat.
To counter this problem, such an attempt has been made as the package material is changed to one that has higher heat resistance (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-156704), or a heat dissipating structure is provided to prevent the heat generated by the light emitting element from building up in the package (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-49442). Heat dissipation from the light emitting element can be improved also by eutectic bonding by means of a soldering material instead of using a die bonding resin when die-bonding the light emitting element to the lead frame (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-49442, Japanese Unexamined Patent Publication (Kokai) No. 2005-259972 and Japanese Unexamined Patent Publication (Kokai) No. 2005-353914).
It is also known to extract light efficiently from the light emitting element by applying silver plating to the surface of the lead frame (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-156704 and Japanese Unexamined Patent Publication (Kokai) No. 2005-353914). This method is expected also to achieve an effect of suppressing the housing temperature from rising, as the reflectivity of the lead frame is improved by the silver plating resulting in light being suppressed from being absorbed in the light emitting device.
While discoloration and deformation can be significantly suppressed by forming the package from a heat resistant material, it is not enough to counter the effect of the output power of the light emitting element currently available, and the package gradually discolors as the device is used longer. When the package discolors, the amount of light absorbed by the package sharply increases. As the absorbed light turns into heat, temperature of the package rises further, thus feeding a vicious cycle of discoloring the package. As the discoloration of the package proceeds, intensity of light emitted by the light emitting device significantly decreases and makes it necessary to replace the light emitting device. This means that it is important to minimize the discoloration of the package in order to elongate the service life of the light emitting device.